1. Field of the Invention
The present invention relates to an alternative non-invasive method and device to monitor cardiac parameters.
2. Description of the Prior Art
U.S. Pat. No. 7,054,679, NON-INVASIVE METHOD AND DEVICE TO MONITOR CARDIAC PARAMETERS issued to Hirsh in 2001, describes a vector space, whose basis vectors are non-invasively obtained in a substantially easy manner. In particular, the vector spaceN={T, MAP, EI, DI, E-M}is described, where T is the Cardiac Period, MAP is the Mean Arterial Pressure in a peripheral artery, preferably the Radial Artery, and E-M is the Electrical-Mechanical Interval. In one embodiment, the E-M interval is measured as the time interval between a signal electrical event in the EKG and a concomitant, causally related mechanical event in the peripheral (radial arterial) pulse wave. EI is the Ejection Interval, the time interval between the opening and the closing of the Aortic Valve. DI is the Diastolic Interval, the time interval between the opening and the closing of the mitral valve.
In one preferred embodiment, the electrical event is measured as the time of the peak in the amplitude of the second derivative of the EKG such as in Einthoven's Lead II. This corresponds roughly to the Q-wave trough in the EKG, which in Lead II has a negative-going depolarization. It is the time at which the EKG depolarization voltage is accelerating maximally upwards. Call this event TE. 
In one preferred embodiment, the mechanical event is the time of the peak in the value of the second derivative of the arterial pressure wave. This is the instant in time at which the pressure wave is accelerating maximally upwards. It corresponds roughly to the time at which the arterial pressure wave starts to take off from its lowest value at the beginning of systole. Call this event TM The Electrical-Mechanical Interval,E-M=TM−TE  eq. 1
This E-M interval has many interesting properties. For example, it can be used to predict the contractile state of the myocardium. If the contractility of the myocardium is described by the quantity dP/dtmax, which is the maximum value of the first derivative of Left Ventricular Pressure as denoted by P during systole, we can say thatln(dP/dtmax)=k(1/E-M)+c  eq. 2where ‘k’ and ‘c’ are constants of proportionality. The natural log of dP/dtmax is linearly proportional to 1/(E-M). This is taught by U.S. Pat. No. 7,054,679.
Similarly, there exists a set of transformation functions, which operate on the quantities contained in N. U.S. Pat. No. 7,054,679 teaches some, preferred embodiments of such functions. Those functions of N in turn, represent a new set of quantities, also expressed as a vector, in a new vector space I. The set of quantities in the vector I have the extremely useful property that they linearly track with the set of hemodynamic quantities that are historically and conventionally obtained only in invasive manners at considerable expense and risk to the patient. These hemodynamic quantities include {Preload, Afterload, Contractility, Stroke Volume, Cardiac Output, End-Diastolic Left Ventricular Myocardial Compliance}.
Using the formalisms of Linear Algebra, we can place appropriately constructed transformation functions into an appropriately constructed diagonal matrix and use that matrix to operate on the vector in N to obtain a vector in I. Significantly, any possible hemodynamic state is described uniquely by exactly one point in N. We can say that N ‘spans’ cardiovascular space. Every point in N is mathematically mapped on to exactly one point in I. There exist no unmapped points in N. The mathematical mapping is ‘one-to-one’, as well as ‘onto’. Likewise, any possible hemodynamic state is described by exactly one point in I.
Moreover, U.S. Pat. No. 7,054,679 teaches that it is possible to display the quantity                {Preload, Afterload, Contractility}which is approximated by        {Left Ventricular End-Diastolic Pressure, Systemic Vascular Resistance, dP/dtmax}as a vector in a three dimensional Cartesian space in real time to be displayed on a computer screen. Each of the three quantities is represented as a component vector along one of three mutually perpendicular axes. This is done simply by using the appropriate mathematical transforms on the appropriate non-invasive quantities in N, as in U.S. Pat. No. 7,054,679. Such a real-time, noninvasive display is very empowering to clinicians and allows even inexperienced clinicians to visualize and to understand hemodynamic and physiological states of their patients as they undergo surgery or as their disease process evolves through time. It can empower healthcare workers to deliver better care at lower cost with considerably less risk to the patient.        
A problem with the non-invasive hemodynamic monitoring approach described in the above has to do with the nature of the E-M interval. While the relation between the E-M interval and the ratio between Stroke Volume and the Ejection Interval (SV/EI) is practically invariant over logarithmic ranges of Systemic Vascular Resistance (SVR) and Left Ventricular End-Diastolic Pressure (LVEDP), the E-M interval is difficult to standardize. If the anatomical position of the arterial pressure detector is controlled for, say by placing it at the radial artery, there remains the problem of normalizing the E-M by the patient's height, arm-span, or some other feature of the patient's physiognomy. While this can be easily accomplished, there also remains the problem of correcting the E-M for
1) the effects of a decrease in the speed of signal transduction by the Purkinje fibers in the cardiac conduction system and the myocardium itself with increasing age, and
2) the decrease in the elasticity of the artery as the patient ages, or as a consequence of illness, and its shortening effect on the time of flight of the pulse wave from the aortic valve to the pressure detector.
The above described problems necessitate the development of a nomogram based on factors such as height, and age, and the presence of disease that might be used to predict the coefficients and constants of linear proportionality (k, c). Furthermore, a nomogram is used for calibration of the various relations described in U.S. Pat. No. 7,054,679 such asexp(1/E-M)=k(SV/EI)+c  eq. 3Such a nomogram would be based on studies of large populations of patients. It could be used within reasonable limits of statistical probability. Alternatively, some other well-accepted invasive or minimally invasive method could be used to calibrate the system for each individual patient on a one-time basis to establish a base line. Unfortunately, these procedures may need to be performed while the patient is under anesthesia or deep sedation. The utility of said calibration could persist for months to years, barring some significant evolution in the natural history of the patient's disease.
Since U.S. Pat. No. 7,054,679 was conceived, three new technologies have emerged that allow for the non-invasive measurement of the Ejection Interval (EI) during Systole and of the Stroke Volume. The first is Esophageal Doppler technology such as the Hemosonic 100 or the Deltex device. The Hemosonic 100 uses an esophageal Doppler and A-mode ultrasound to measure the diameter of the descending aorta as a function of time as the pulse wave traverses it and then integrates the cross-sectional area of the descending Aorta times the cross sectional blood velocity over the ejection interval to yield the stroke volume (SV). The Deltex device simply measures the average blood flow in the descending Aorta and yields a ‘stroke distance’ over the ejection interval that does not correct for variations in the cross sectional area of the aorta.
The second is Pulse Contour technology, which approximates the (SV) from the peripheral arterial pulse pressure that is measured invasively using an indwelling arterial catheter. In principle, the Pulse Contour method can be used with the trace of the non-invasively measured pulse wave, using the T-line from Tensysmedical. One example of pulse contour technology is the Edwards Vigilio System, and LiDCO, which is an indwelling Lithium ion electrode used to calibrate the system with cardiac output information. Another is the Pulsion PiCCO system by Phillips.
The third is Impedance Cardiography technology. This technology injects a high frequency, low milliamp current into the chest from two or more skin electrodes. Two or more skin electrodes are used immediately inside the current-injecting electrodes. A voltage is measured across these sensing electrodes, and impedance is calculated as it varies with the cardiac cycle in a complex way. Stroke Volume and Ejection Interval are also calculated from the waveform based upon proprietary algorithms on a beat-to-beat basis. Examples include the IQ AOE Impedance Cardiography System, and the Physioflow device.
A second problem with the E-M interval as derived from the EKG signal and the peripheral arterial pressure wave is that the beat-to-beat measurement of the arterial pressure wave has required the use of an invasive indwelling peripheral arterial catheter. Although the use of such a catheter is routine, it has been associated with patient injury including rare but catastrophic loss of a patient's hand. Another technology, which has emerged since 2001 and already alluded to, is the T-line by Tensysmedical, which obviates the above problem. This device gives the clinician a non-invasively obtained radial arterial pressure wave that is the physiological equivalent of an invasive arterial pressure wave signal. It works by the use of a piezo-resistive element configured in a Wheatstone bridge circuit. The piezo-resistive element is held over the radial artery by means of a spring loaded clamp. A first servomotor then moves the piezo-resistive element over the width of the ventral side of the wrist in the ‘x’ direction to find the maximum amplitude of the radial arterial pulse. A second servomotor moves the piezo-resistive element vertically in the ‘z’ direction bringing a servo-controlled pressure to bear upon the circular cross section of the radial artery. It adjusts itself in the ‘z’ direction until it gets a maximal amplitude of the pressure indicative of the mean arterial pressure inside the radial artery. Sinusoidal deviations from this pressure during the pulse wave are displayed as a pressure wave on a suitable display. By using this device in conjunction with an EKG signal, it is possible to create a physiologically useful E-M interval as described in U.S. Pat. No. 7,054,679, and the E-M interval stands in a physiologically useful relation to dP/dtmax and SV/EI.
These new technologies put a real time beat-to-beat data stream of EI and SV easily within the grasp of clinicians. They do not depend on the time-honored methods of thermodilution using a Swan-Ganz catheter and thereby avoid the level of trespass and risk of injury associated with this device.